Emails go missing randomly when I have seen them in my inbox recently

Situation

Outlook desktop set as follows

 - IMAP/POP account for BT mail (used most as had bt email address forever!)

     - gmail account diverted completely via bt account

 - hotmail account for microsoft

Symptoms

If the random email comes in 'fresh' I can see it. 

If I happen to click on my sent folder and go back to my inbox, it has 'dissapeared'

The only way I can see it again is to close and open outlook again.

Very confused as this has not happened before?

Thank you

Graham